CDC turns to social sites to get flu message out

"If you get sick and you get a fever get help promptly. But don't overwhelm the emergency department with people who are not very sick," Frieden said.

CDC officials have been urged to admit when they do not know something.

"The only thing that is certain is uncertainty," Frieden added.

The CDC is also aware that some people doubt the safety of vaccines.

"People do have some trust issues. That's 2009. At CDC we are really committed to accurate, timely information," said the CDC's Dr. Anne Schuchat, director of the respiratory disease section.

Vaccine safety groups argue that insufficient testing of swine flu vaccine could pose a risk to the public and say individuals will have little legal recourse if problems emerge.

But paradoxically, public interest in the pandemic offers a chance to present concerns about vaccinations to a wider public, said Barbara Fisher, president of the National Vaccine Information Center.

"It seems irresponsible to advocate and promote a vaccine and have no safety net whatsoever, and so yes it seems like an opportunity to point to some of the problems," said Fisher, a longtime critic of public vaccination programs.

(Editing by Maggie Fox and Mohammad Zargham)